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\section*{Acknowledgements}
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First of all, I would like express my deepest gratitude to my supervisor Falko Guderian, for his instructive advice, useful suggestions, generous assistance and continual encouragement, which contribute to the completion of my thesis.

Furthermore, I want to thank Prof. Dr. Gerhard Fettweis for giving me the opportunity of developing this thesis at the Vodafone Mobile Communications Chair, for insightful discussions and his support.

Finally, I am indebted to Ying Su for her continuous support and encouragement.




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